The present invention relates in general to continuous control valves and in particular to a new and useful continuous control valve in which various flow rate displacement characteristics are adjustable and in which a definite rate of flow of hydraulic fluid under a definite pressure is continuously controllable in accordance with a continuously and smoothly rising flow force to flow rate characteristic.
Continuous control valves, particularly those comprising a cylindrical body or slide in which grooves with a rectangular cross section are provided, are known both in the rotary slide and the lengthwise or axial slide design (for example of rotary slide see German OS No. 27 30 652; and for lengthwise slide see German OS No. 24 04 524). Usually, and unlike in the last mentioned reference, the rectangular grooves extend in their length, parallel to the axis of the slide casing and to the axis of symmetry of the cylindrical slide coinciding therewith. The length of the grooves is determined by the spacing of the inside openings of adjacent ports. In a four-way valve, two grooves are needed which extend on diametrally opposite sides of the cylindrical slide.
It is further known (from British patent No. 134,759 FIGS. 1 and 2, and from U.S. Pat. No. 3,477,472, FIG. 1) to provide the control piston of control valves with a step in order to reduce the flow forces, or to provide such a step close to the control edge of the control piston. Continuous control valves of the above mentioned kind are type provide means for a continuous control in most various applications. These control means become increasingly important as transducers or transformers, and at the same time amplifiers, in a so called pneumo-hydraulic control system. This system has been developed in the paper and textile industries for controlling the run of material web edges or center lines. The basic element of this system is a pneumo-hydraulic controller, comprising a pneumo-hydraulic transformer and a unit for supplying pressure oil and compressed air.
The pneumo-hydraulic transformer, designed as a servo-valve, comprises a double-diaphragm which is exposed to a differential pressure corresponding to the deviation from a desired position, for example, of a web edge. Under the differential pressure, a diaphragm drive executes a lengthwise stroke and causes a corresponding rotary motion of the rotary slide of the servo-valve, whereby a working oil flow is continuously controlled to reduce the deviation of the web through an actuator.
A valve for continuous control or a continuous control valve designed and employed as a part of a pneumo-hydraulic transformer or for other purposes, has a definite flow rate-displacement characteristic determining the control characteristic. Therefore, to obtain continuous control valves with varying characteristics, the slide and/or slide casing must be manufactured in various sizes. This entails the necessity of manufacturing many valve sets in small numbers since valves of this kind are sometimes needed with a rotary slide and other times with a lengthwise slide and with considerably differing characteristics. On the other hand, if a continuous control valve is designed as a pneum-hydraulic transformer, its property and function as an amplifier is also utilized, to be able to control a hydraulic power output (output of the continuous control valve) which is greater than, for example, the force actuating the rotary slide (input power). The mentioned diaphragm drive of the pneumo-hydraulic transformer designed as a servo valve has only a small actuating power, so that the power input of the servo valve is small.
Continuous control valves in which steps, similar to those of the above mentioned prior art are provided at the flow entrance, downstream of the control edges, for compensating or reducing the hydraulic flow force, are already commercially available and employed. At these steps, a "compensating force" acting against the flow force tending to close the valve is produced, so that a smaller force for controlling the slide and a smaller control power input are needed than in valves without a flow force reduction, provided that the considered power output is identical. Prior art methods of reducing the flow force in valves are not satisfactory in this regard.